The earth is an Oblate Spheroid which means it is a sphere which is slightly flattened at the poles and slightly bulged at the equator. For navigation purposes it is considered a perfect sphere with some important features which form the foundation for the coordinate system used for determining positions. A basic knowledge of the earth coordinate system is a fundamental to understanding the earths coordinate system.
The first, is the axis of the earth's rotation. If you imagine the axis as a straight line passing through the center of the earth, the two points where it passes through the surface are called the North and South terrestrial poles.
Another fundamental reference is established by passing a plane through the center of the earth, perpendicular to the axis of rotation. This action is similar to cutting a grapefruit in half with a knife, and will be a line on the surface of the earth exactly halfway between the poles. This line is called the equator. The equator cuts the earth into two equal halves called the Northern and Southern Hemispheres. The equator is an example of a great circle. A great circle is formed by any plane which passes through the center of the earth and divides it into equal hemispheres. Distance can be measured along great circles because each degree of arc along these circles equals 60 nautical miles. Since each degree contains 60 minutes, each minute of arc along a great circle equals one nautical mile. Planes which pass through the earth's surface, but not through the center of the earth, are circles on the surface which are called small circles.
Planes which pass through the earth's surface, but not through the center, parallel to the plane of the equator, are small circles called parallels. Parallels are the top and bottom borders and horizontal lines printed on charts used in piloting. A parallel is located in terms of the number of degrees, minutes, and tenths of minutes of arc measured between the equator and the parallel.
Parallel is another name for latitude, and is termed north if measured from the equator toward the North Pole, and south if measured toward the South Pole. Latitude is zero at the equator and 90 degrees North or South at the poles. The pole of the hemisphere in which an observer is located is called the elevated pole. The elevated pole is always the pole which is nearer to the observer which means the North Pole for an observer in north latitude, and the South Pole for an observer in the Southern Hemisphere. Latitude provides one of the coordinates needed for locating positions on the earth's surface, but a second coordinate is needed so you can pinpoint a spot on a parallel of latitude. To do this a universally accepted starting point for measurement similar to the equator in measuring latitude had to be established. It was agreed that the great circle described on the earth's surface by passing a plane through the center, both poles, and the Royal Observatory at Greenwich, England would be the starting point for a measurement called longitude.
All great circles passing through both poles are called meridians. Meridians are represented by the side borders and the vertical solid lines printed on charts used in piloting. Since these meridians are also great circles, you can measure distance using the latitude scale which is printed along the side border. The meridian of zero degrees longitude is called the Greenwich or prime meridian. The Greenwich meridian is actually the upper branch of the Greenwich meridian because it is the half of the meridian which has the poles as end points and passes through Greenwich. The other half of the Greenwich meridian, called the lower branch, is called the 180th meridian. Another way of describing this is that the upper branch of a meridian is that half which extends from the North Pole to the South Pole on the same side of the earth as the observer is located. The lower branch of any meridian is at the longitude which is 180 degrees from the meridian on which an observer is located.
Longitude is measured eastward or westward from the upper branch of the Greenwich meridian to the meridian passing through a given point. It is equal to the angle between the the Greenwich meridian and the meridian of the place measured at the pole. This angle cannot exceed 180 degrees and is called "west" if the point in question is located between Greenwich and 180 degrees west. If the point lies east of Greenwich, but less than 180 degrees east, it is in east longitude. The180th meridian is called both180 east and west.
Each degree of latitude or longitude is divided into sixty minutes. But only minutes of latitude equal 1 nautical mile, because latitude is measured along a meridian which is a great circle. Each minute is divided into sixty seconds or tenths of minutes. Division of minutes into tenths is the most common practice. Positions are given with latitude first followed by longitude such as Lat. 44°36.8' N, Long. 124°06.3' W.
